The present invention relates to a data receiver for use in a system in which source data can be produced at different data rates, but the data are interleaved and transmitted at a single, fixed rate.
Examples of such systems can be found among cellular telephone systems employing code-division multiple-access (CDMA) communication. Some of these systems use a variable-rate vocoder which can encode a digitized voice signal at several different output data rates, higher rates leading to higher speech quality when the signal is decoded. The coded signal is transmitted at a rate equivalent to the highest output rate of the vocoder. When channel conditions are unfavorable, the vocoder output rate is reduced, and each output bit or symbol is transmitted multiple times to increase the probability of successful reception.
It therefore becomes necessary for the receiver to identify the data rate of the received signal. A conventional approach is to carry out the usual receiving process at each rate in turn until correct results are obtained, as determined from check information that was inserted into the transmitted data. This approach requires much processing, however. In the worst case, every one of the data rates must be tested before the correct rate is found.
Under unfavorable channel conditions, even after the data rate has been correctly identified, there remains the problem of data corruption in transmission. A conventional practice is to combine all of the received data by summing or averaging, in the expectation that errors will be smoothed out. A problem is that when large errors occur, due to a burst of noise, for example, the contribution of erroneous symbols to the sums or averages may outweigh the contributions of other symbols to such a extent that incorrect data values are obtained.